A pair of conductive lines, such as a signal conductor and a signal-return conductor, are coupled when they are spaced apart, but spaced closely enough together for energy flowing in one to be induced in the other. The amount of energy flowing between the lines is related to the dielectric medium the conductors are in and the spacing between the lines. Even though electromagnetic fields surrounding the lines are theoretically infinite, lines are often referred to as being closely or tightly coupled, loosely coupled, or uncoupled, based on the relative amount of coupling. The amount of coupling may be defined by a coupling coefficient. However, as a practical measure, two lines may be considered to be inductively coupled when a detectable signal is coupled from one line onto the other. A threshold of coupling may be appropriate to distinguish between coupled and uncoupled lines. In most applications, two lines that have less than 20 dB inductive coupling between them are considered to be uncoupled lines, and are coupled lines otherwise. In some applications, lines that have less than 100 dB are considered to be uncoupled lines. In terms of a coupling coefficient, two lines may be considered to be closely coupled if the coupling coefficient is 0.1 or greater. Thus, two lines may be considered as loosely coupled or substantially uncoupled if they have a coupling coefficient of less than 0.1.
Transmission-line networks, such as impedance-transforming combiners (including dividers) and transformers, typically include such coupled lines, but may be restricted to limited frequency ranges of operation due to resonance inherently in their topologies. Some of these combiners/dividers and transformers may possess bandwidths with bandwidth ratios as great as 50:1 even when limited by inherent resonance.
A common characteristic of transmission-line networks, like impedance-transforming combiners and transformers, is the construction of junctions. Such junctions are formed by merging two or more transmission lines at a common point or junction. Where two transmission lines are merged the junction may be configured in the shape of a letter T, thus giving rise to the name T-junction. Under ideal conditions, when the two original transmission lines are strictly identical, usually a high Q resonance occurs at a single frequency. However, under realistic manufacturing conditions in which the two original lines are not exactly identical, the frequency response of a conventional T-junction is subject to a more pronounced (low Q) resonance.